Constants saved for reference

The SI is defined in terms of a set of seven defining constants. The complete system of units can be derived from the fixed values of these defining constants, expressed in the units of the SI.

The seven defining constants are:

the caesium hyperfine frequency ΔνCs
the speed of light in vacuum. c
the Planck constant. h
the elementary charge. e
the Boltzmann constant. k
the Avogadro constant. NA,
the luminous efficacy of a defined visible radiation Kcd.

It is by fixing the exact numerical value of each that the unit becomes defined, since the product of the numerical value and the unit must equal the value of the constant.

Their numerical values and the units they define are as follows:

Defining constant |Numerical value| Unit

ΔνCs 9 192 631 770 Hz
c 299 792 458 m s–1
h 6.626 070 15 x 10^–34 J s
e 1.602 176 634 x 10^–19 C
k 1.380 649 x 10^–23 J K–1
NA 6.022 140 76 x 10^23 mol–1
Kcd 683 lm W–1


The numerical values of the seven defining constants have no uncertainty.

The definitions of the base units specify the exact numerical value of each constant when its value is expressed in the corresponding SI unit.